Upgrading using pkg_add applies to both packages and ports, as explained in FAQ 15. Upgrades are only relevant to upgrades from one -release to the next. e.g. if you upgrade from 5.5-RELEASE to 5.6-RELEASE and have updated your $PKG_PATH accordingly this enables you to upgrade the binary packages to those included with the new release.

FAQ 5.1, on "flavors" of OpenBSD, is an important document to read. It will help you to understand how OpenBSD is developed and released.

Releases are always permanent, fixed binary files. The Project does not ever modify them later.

You can choose to apply any or all errata patches, or, as mentioned, you can build a -stable system, which includes all errata patches, plus any minor patches that do not qualify for errata publication.

The Project commits "-stable" third party ports, but in recent years has not created -stable packages as the Project doesn't have the resources to do so for all of its architectures.

A third party, M:Tier, offers -stable systems and packages to the OpenBSD community.